1. Field of the Invention
The present invention relates to a color image forming apparatus.
2. Description of the Related Art
There has been known a color image forming apparatus (so-called tandem-type color image forming apparatus) having a plurality of drum-type photoconductors. In the color image forming apparatus, it is important to suppress the positional deviation for every color (color misregistration) to an unnoticeable degree. When the color misregistration is great, it might be evaluated that the image quality is deteriorated. The greatest factor of the color misregistration is a periodic crude density on the output image caused by the eccentricity of each photoconductor. The ideal countermeasure is that the eccentric amount of each photoconductor is sufficiently reduced, but trade-off between cost and mass-productivity should be considered.
In view of this, various ideas have been provided to make the color misregistration unnoticeable even if the eccentric amount is the same. For example, an apparatus in which the peripheral length of each photoconductor drum and the peripheral length of the transfer belt are set to have ratios of whole numbers has been proposed (for example, Japanese Patent Laid-Open No. 7-261499).
When the phases of pitch fluctuations caused by the eccentricity of each photoconductor are not matched on the output image, the color misregistration becomes noticeable. This point is focused, and various ideas have been given for matching the phases of eccentricity of each photoconductor on the output image so as to make the color misregistration unnoticeable. In this case, in order to detect the rotational phase of each photoconductor, a toner pattern (toner image) having lines, parallel to the rotational axis of the photoconductor, arranged at equal spaces in the rotating direction is formed, and the deviation from the expected position is detected.
Alternately, a photoconductor that stores a pulse pattern for canceling a speed fluctuation of its one rotation, thereby driving a stepping motor and reducing the pitch fluctuation caused by the eccentricity, is known (for example, see Japanese Patent Laid Open No. 63-75759).
In addition, a photoconductor that applies fine adjustment individually to the rotation speed of a rotor so as to cancel a fluctuation thereof, by information of vibrating component regarding a periodic rotational fluctuation, is known (for example, see Japanese Patent Laid Open No. 10-78734).
Usually, the color image forming apparatus performs the image formation by using three primary colors of yellow, cyan and magenta, and black. The tandem-type image forming apparatus includes four photoconductors corresponding to each color. In the case of the monochromatic image formation, only the black photoconductor is used.
In such an image forming apparatus, when a ratio of occupancy of the monochromatic image formation to an entire body is large, only black photoconductor is deteriorated rapidly. In this case, unbalance is generated at a maintenance time of each photoconductor of monochromatic color and others (yellow, cyan, and magenta). Therefore, a standard ratio of monochromatic image formation and color image formation is previously estimated at the time of designing, and in accordance with the estimated ratio, the service life of the photoconductor is set.
Further, there is an image forming apparatus that prevents other photoconductors from being actuated, at the time of the monochromatic image formation. By doing so, this image forming apparatus is capable of preventing a deterioration of the photoconductor and a developer not contributing to image formation. In addition, this image forming apparatus is capable of setting a moving speed (process speed) on a photoconductor surface at the time of monochromatic image formation faster than the moving speed at the time of color image formation, thereby also setting its print speed faster.
From the viewpoint of prolonging the service life of the black photoconductor and setting the process speed faster, it is preferable that the diameter of the photoconductor is increased. However, if only the diameter of the black photoconductor is greater than the diameter of the other photoconductors, various subjects involved with the color image formation arise.
The representative one is the subject relating to the color misregistration. Since the rotational cycle of the black photoconductor is different from those of the other photoconductors, the technique for matching the direction of the eccentricity to make the color misregistration unnoticeable cannot be taken. Meanwhile, in the case of generating correction patterns of the number of the photoconductors to cancel a speed fluctuation of one rotation of the photoconductor, the configuration is complicated and the cost is disadvantageously increased in most cases.
A technique for making the color misregistration unnoticeable with a simple configuration has been desired even in case where a plurality of types of photoconductors, each having a different diameter, are used.